The three main probes for functional vWF activity--ristocetin, botrocetin, and the PAggF test--and similarities and differences in their elicited vWF activities have been reviewed. Emphasis has been placed on the technologies dependent on these probes, with a brief description of a series of relatively simple and sensitive tests developed in this laboratory. These tests include the development of the PAF test for vWF in certain animal plasmas; the development and use of fixed lyophilized platelets that retain receptor activity for vWF; the purification of botrocetin (venom coagglutinin) freed of thrombinlike enzymes and its use in vWF assays; the development of macroscopic platelet aggregation tests for screening and assay of vWF; and the application of the macroscopic test for rapid screening and quantitation of human plasmas for acquired inhibitors of vWF utilizing each of the three probes. Historically, the similarities of the ristocetin and botrocetin probes were first observed. For normal human plasmas and for patients with classic vWD, both homozygous or heterozygous, similar values for vWF were obtained with these two probes. Similar platelet binding of vWF in the presence of the two probes was likewise noted. However, further studies of these two probes revealed striking differences. Especially important for study of animal plasmas generally as well as a canine model of vWD was the observation that the vWF in all animal plasmas tested with botrocetin was highly reactive, whereas with ristocetin nearly all plasmas were resistant. Similarly, all animal platelets tested for vWF-dependent aggregation with the two probes were highly reactive with botrocetin, but inactive with ristocetin.(ABSTRACT TRUNCATED AT 250 WORDS)